Method of acquiring images at a plurality of acquisition locations of an acquisition device

ABSTRACT

A method of acquiring images includes moving, into a plurality of acquisition locations, of an acquisition device including at least one camera, and acquisition at each acquisition location of at least one image of a scene by the camera. Each acquisition location being chosen in such a manner that scenes viewed by the camera at two consecutive acquisition locations and corresponding images overlap, at least partially, and areal density of pixels assigned to at least one element of the corresponding scene, which is represented in the corresponding image by a high-resolution portion, is greater than 50% or greater than 80% of a target areal density, the areal density of pixels being defined as a ratio of the area of the element projected in a plane perpendicular to an optical axis of the camera over a quantity of pixels of the high-resolution portion.

The invention relates to a method for acquisition of images of scenes ofan environment and a method for constructing a 3D digital model from theimages.

In order to display a digital model of a scene including a set of real3D objects, for example on a screen or by means of an augmented realityhelmet, a known solution is to construct a cloud of points whichdiscretize the outer envelope of each of the objects.

For this purpose, a known method consists in acquiring images by meansof one or more cameras disposed at various acquisition locations and/orin various directions of observation, then in generating the cloud ofpoints from the images, for example by stereoscopy.

The problem associated with this method is that, although it is capableof rendering details of the objects disposed near the camera with a goodresolution, those disposed far away are, on the other hand, reproducedwith a lower resolution linked to their distance from the camera.Indeed, the resolution of a pixel, which is proportional to the inverseof the dimension of a part of an object which is imaged by a pixel,decreases as the distance between the camera and the object increases.Thus, for a given definition of image, defined as the number of rows andof columns of pixels defining the image, the image may comprise pixelsof different resolutions depending on the distance between the camerahaving acquired the image and the object or objects to be imaged.

Thus, when the 3D digital model is displayed, a user may observe avirtual body, generated for the displaying of the digital model, whichmay appear to them to be poorly visually detailed, even when virtuallygoing very close to the virtual body, this body representing an objectsituated far from the camera when the images were acquired.

The invention aims to overcome the drawbacks described hereinabove and,for this purpose, it provides a method of acquiring images comprisingthe moving, into a plurality of acquisition locations, of an acquisitiondevice comprising at least one camera, and the acquisition, at eachacquisition location, of at least one image of a scene by means of thecamera,

-   -   each acquisition location being chosen in such a manner that:    -   the scenes, viewed by the camera at two consecutive acquisition        locations, and the corresponding images are at least partially        overlapping, and    -   the areal density of pixels assigned to at least one element of        the corresponding scene, which is represented in the        corresponding image by a high-resolution portion, is greater        than 50%, preferably greater than 80% of a target areal density,        the areal density of pixels being defined as the ratio of the        area of the element projected in a plane perpendicular to the        optical axis of the camera over the quantity of pixels of the        high-resolution portion.

A bank of images comprising images of the acquired scenes can thusadvantageously be formed, and it is thus ensured that each image of thebank comprises a high-resolution portion. The acquisition methodaccording to the invention thus renders possible the creation of a 3Dmodel of an environment with a substantially uniform resolution, bybasing itself on the high-resolution portions of each image and byputting aside the portions defined with a lower resolution.

When the 3D model is displayed, an observer, who virtually moves betweenthe generated bodies, can thus observe them with substantially the samelevel of detail.

The high-resolution portion preferably represents more than 20% of thetotal number of pixels defining the image. It may represent less than100%, less than 90%, less than 80%, or even less than 50% of the totalnumber of pixels defining the image. The image may comprise alow-resolution portion which represents the complement, in number ofpixels of the image, of the high-resolution portion.

The areal density of pixels assigned to the element of the correspondingscene that is represented in the corresponding image by thehigh-resolution portion may be greater than the target areal density.The areal density of pixels assigned to an element, for example anobject, of the scene represented in an image corresponds to the numberof pixels representing the element in the image, divided by the area ofthe element. It may be expressed in px·mm⁻² [pixels per squaremillimeter]. For example, in an image having a high-resolution portionin which an object is represented with N pixels, the areal densityassigned to the element is N pixels per square millimeter of surface ofthe element.

The “target areal density” is determined by the operator implementingthe acquisition method, for example in a prior step or during theacquisition.

For the sake of being concise, the areal density of pixels assigned toat least one element of the scene, which is represented in thecorresponding image by a high-resolution portion, is denoted “arealdensity of pixels of the high-resolution portion”.

A “scene” is a part of an environment that may be observed by an imageacquisition device when the latter is immobile at a given acquisitionlocation. A scene may comprise at least one, preferably several,elements, notably an object. The object may be a structure, for examplea bridge or a tunnel, a building, a house, a stadium, a ship, awarehouse, an urban infrastructure, for example a bus shelter, anornamental article, a road sign, a traffic light, a wall, a side forexample of a tunnel, or a part of the latter. The object may be anatural element, for example a tree, a rock, a mountain, a wall of acave.

Furthermore, the operator knows in a routine manner how to determine theacquisition locations in order to ensure that the areal density ofpixels of the high-resolution portion is according to the invention.

As illustrated schematically in FIG. 1, a camera 5 comprises a lens 10and a sensor 15 having photosites 20. Each photosite receives a portionof the radiation 25 coming from an object 30 to be imaged passingthrough the lens and converts it into information, generallycolorimetric data, to be assigned to a corresponding pixel in the image.

The areal density ρ associated with an element of a scene in an imagedepends on the focal distance f of the lens, on the distance D betweenthe main object plane 33 of the lens of the camera and the element, andon the size d_(px) of the photosite on the sensor of the camera defininga pixel. It is expressed by the following equation (1):

$\begin{matrix}{\rho = \lbrack \frac{f}{d_{px}( {D - f} )} \rbrack^{2}} & (1)\end{matrix}$

A person skilled in the art can easily determine, knowing the focallength of the objective and the size of the photosite, the areal densityassociated with an element of a scene, by simply measuring the distancebetween the element and the acquisition device.

For example, when the main object plane of the camera is disposed at adistance D equal to 5 m from the nearest object of a scene, for a sizeof photosite d_(px)=1 μm and for a focal distance of the camera lens of50 mm, the areal density ρ associated with the object is around 102px·mm⁻².

Thus, after having determined an areal density ρ_(min) of ahigh-resolution portion of an image to be acquired, those skilled in theart know that they have to position the acquisition device with respectto the nearest object in such a manner as to ensure that the distance Dfulfills the following condition (2):

$\begin{matrix}{D \leq {f( {1 + \frac{1}{\sqrt{\rho_{\min}}d_{px}}} )}} & (2)\end{matrix}$

In other words, a person skilled in the art, after having chosen thetarget areal density he wishes to achieve, for example knows thedistance, measured between an element of the scene and the acquisitiondevice, below which the element will be represented with an arealdensity of pixels greater than the target areal density.

The method may therefore comprise, prior to the acquisition of theimages, the measurement of the distance between at least one element ofthe scene to be acquired and the main object plane of the camera lens,and the verification of the condition (2). The measurement is forexample carried out by means of a rangefinder, for example using lasersighting, mounted on the acquisition device.

Preferably, the target areal density is greater than or equal to 50px·mm⁻²[pixels per square millimeter], preferably greater than or equalto 60 px·mm⁻², or better in the range between 70 px·mm⁻² and 100px·mm⁻². A 3D model may thus be constructed with a high resolution,notably close to or superior to that which the human eye is capable ofdetecting. An observer can thus observe in the display of the 3D modelthe same details as if they were observing the environment, from thesame point of view.

The areal density of pixels may be greater than 90%, or better greaterthan 95% of the target areal density. It may be less than 110%, notablyless than 105% of the target areal density.

The scenes viewed by the camera in two consecutive acquisition locationsand the corresponding images overlap, at least partially. Theoverlapping of the images facilitates the discretization of the scenes,notably by photogrammetry. Preferably, the overlap fraction between saidimages is greater than 70%, or even greater than 80%, for examplegreater than 90%. The overlap fraction may be calculated:

-   -   optionally, by projecting one of the two images into a plane the        normal of which is parallel to the viewing axis of the camera in        the position in which the other image has been taken, then    -   by matching, for example by correlation of images, the image,        optionally projected, and the other acquired image, so as to        determine the common area between the image, optionally        projected, and the other image, and    -   by expressing as percentages the ratio of the number of pixels        in the matched area over the number of pixels in the other        image.

For example, the projection step described hereinabove is carried outwhen the angle between the viewing axes of the camera at the consecutiveacquisition locations is greater than 5°.

Preferably, the acquisition device comprises a plurality of camerasdisposed for acquiring at the same acquisition location respectiveimages which overlap one other. The acquisition device may notablycomprise at least two cameras, preferably at least five cameras,preferably at least ten cameras, or better at least fifteen cameras, thecameras each acquiring a corresponding image of the scene along adifferent viewing axis from the others, the images thus acquired of thesame scene overlapping one another. In this way, the construction of the3D model is facilitated, since these images can be more easilycorrelated with one another, for example by photogrammetry.

The cameras may be synchronized in such a manner as to acquire images atthe same moment. The subsequent processing of the images is thusfacilitated.

Preferably, the acquisition device comprises an acquisition modulecomprising a monopod and at least two acquisition stages disposed atdifferent heights on the monopod, each acquisition stage comprising aplurality of cameras each configured for acquiring an image of thescene, the viewing axes of the cameras of one acquisition stage beingangularly distributed around the axis of the monopod such that theacquired images angularly overlap. The monopod may be designed to becarried by an operator moving around in the environment. On the lowerpart, it may comprise a foot allowing it to be placed on the ground. Thedevice may comprise at least three, preferably three, acquisitionstages. The cameras of each stage being distributed around thelongitudinal axis of the monopod over a total angular sector in therange between 90° and 120°, preferably in the range between 150° and190°, in particular equal to 180°. The spacing between the acquisitionstages may be adjustable. The cameras of an acquisition stage arepreferably fixed with respect to one another. The acquisition module maycomprise at least six, preferably at least ten, preferably at leasttwelve, notably fifteen cameras.

Preferably, the optical adjustments of the camera are identical at atleast two, preferably at all, the acquisition locations. In particular,the focusing of the camera may be identical at all the acquisitionlocations. Preferably, the aperture of the camera lens and the shutterspeed of the camera are identical at all the acquisition locations. Thedifferences in contrast and in brightness between images of the samescene acquired at various acquisition points are thus limited.

Preferably, the lighting of the scene has a constant intensity at allthe acquisition locations. The colorimetric differences between twooverlapping images acquired at various acquisition points are thusreduced, which facilitates their correlation and the creation of the 3Dmodel. The scene or scenes may be lit by means of at least one lamp thecolor temperature of which is constant, for example in the range between5000 K and 5500 K. The lamp may include light-emitting diodes.

The camera may acquire a film in continuous mode formed of chronologicalsequences of images. It may, as a variant, acquire photographic images.For example, the camera may be portable, for example of the GoPro mark.It may be a still photo camera, for example of the reflex type. It maybe configured for generating a digital image in a standard image dataformat, for example chosen from amongst jpeg, png, tiff, raw and bmp,preferably raw, or for generating a film, for example in the standardformat chosen from between avi, mpeg and mkv, from which thechronological sequence of images may be extracted.

The images may each comprise more than 1 million, or more than 4million, or more than 8 million, or better more than 16 million pixels.

The acquisition device may be such as described in the patentapplication FR 1856591, incorporated here as a reference.

The acquisition method comprises moving the acquisition device in aplurality of acquisition movements.

Moving the acquisition device may be provided by the movement, notablywalking, of an operator handling the acquisition device. As a variant,the acquisition device may be moved by means of a vehicle, for examplean automobile, or of an aircraft, for example a drone, or of a liftingmachine, for example a crane or a winch, on which the acquisition deviceis mounted.

The consecutive acquisition locations may define a terrestrial path,notably urban. The path may be underground, for example following anetwork of sewers or tunnels. It may be aerial, for example when thedevice is mounted on a drone and the images are acquired as a birds-eyeview.

Preferably, the scanning device is moved along a path defined by theconsecutive acquisition locations, then, starting from the lastacquisition location of the path, the scanning device is moved in thedirection of the first location of the path, in such a manner as toacquire scenes different from those acquired when following the path.The number of images acquired is thus increased and, as a consequence,the total number of high-resolution portions. Furthermore, even moreimages of the same element are acquired observed from variousacquisition locations according to various points of view. The laterprocessing of the images, for example by photogrammetry, forconstructing the 3D model is thus improved. The scenes acquired duringthe movement in the direction of the first location include, preferably,objects included in the scenes acquired when the path was followed.

The acquisition device is moved between first and last acquisitionlocations. Preferably, the number of stops between the first and lastacquisition locations is less than 10, preferably less than 5. It wouldbe even better for the movement of the acquisition device to be carriedout without stopping. Preferably, the scanning device then comprises acamera acquiring a film in the form of a chronological sequence ofimages. The total duration of the acquisition of the images between thefirst and last acquisition locations is thus reduced, which thusincreases the productivity of the operator implementing the method.

In particular, the average speed of movement of the acquisition devicebetween the first and last acquisition locations may be greater than 0.4m·s⁻¹. It is thus possible to cover a long path in a reduced time.

For example, the method comprises the moving into at least 10acquisition locations per minute, or at least 60 acquisition locationsper minute, or even at least 100 acquisition locations per minute.

The frequency of acquisition of the images during the movement isgreater than 0.5 Hz, notably greater than 24 Hz, in particular when theimages form a chronological sequence of a film.

The movement may follow a substantially unidirectional path between thefirst and last acquisition locations. For example, the path may followthe direction of extension of a tunnel. As a variant, the path maycomprise numerous changes in direction, for example when the environmentextends over a surface. For example, the path may run along a series ofstreets of a town, oriented in several directions. Within a street, thepath may comprise at least one crossing of the road in order to acquireimages from opposing sidewalks of the street.

Furthermore, in contrast to the 3D models of the prior art which aregenerally obtained using the maximum amount of exploitable informationcontained in the acquired images, the method according to the inventionmakes available, for the later construction of a 3D digital model,images from which only the high-resolution portions may be used,although such images might comprise other usable information. It istherefore preferable, according to the invention, to acquire a largernumber of images and, preferably, at a higher number of acquisitionlocations, with respect to the number of images needed to implement amethod of the prior art.

The path may comprise more than 10 acquisition locations per kilometer,preferably more than 100 acquisition locations per kilometer, or morethan 1000 acquisition locations per kilometer, or even more than 10,000acquisition locations per kilometer, or even better more than 100,000acquisition locations per kilometer, depending on the length of thepath.

The path may comprise more than 10 acquisition locations, preferablymore than 100 acquisition locations, even better more than 1000acquisition locations.

The path may comprise more than 1000 acquisition location per squarekilometer, or more than 10,000 acquisition locations per squarekilometer, or more than 1 million acquisition locations per squarekilometer, or even more than 10 million acquisition locations per squarekilometer and the surface area on which the acquisition locations aredisposed is greater than 1 m², or greater than 100 m², or greater than1000 m², or even greater than 1000 m².

More than 1000 images, preferably more than 10,000 images, or even morethan 100,000 images may be acquired between the first and lastacquisition locations.

Preferably, the same element, for example the same object, is acquiredat least 5 times, or at least 10 times, or even at least 50 times atdifferent acquisition locations. In this way, the number ofhigh-resolution portions representing all or part of the element in thevarious images is increased.

As has already been described, the method according to the inventionproduces a bank of images, each of the images comprising ahigh-resolution portion which may be processed in order to construct a3D digital model.

Thus, the invention furthermore relates to a method for constructing a3D digital model representing at least one object, the methodcomprising:

-   -   the acquisition of a plurality of images between first and last        acquisition locations by means of the method according to the        invention,    -   the selection, for each scene viewed by the camera, of at least        one high-resolution portion in at least one corresponding image,    -   the construction of a 3D elementary model of each scene viewed        by means of at least one corresponding high-resolution portion,        and    -   the assembly of the 3D elementary models in order to produce the        continuous 3D digital model between the first and last        acquisition locations.

Preferably, the selection of the high-resolution portion or portions inthe corresponding image comprises:

-   -   the recognition in the image, for example by correlation of        images, of areas common to the image and to another image        overlapping the image at least partially,    -   the other image being acquired by the camera at another        acquisition location or being acquired by another camera of the        acquisition device at the same acquisition location,    -   the calculation of the distance between the main object plane of        the camera such as disposed at the acquisition location of the        image and the element or elements of the corresponding scene        represented on the common areas of the image, for example        according to a stereoscopic calculation algorithm, in such a        manner as to determine the areal density of the element or        elements of the corresponding scene, and    -   the selection of the portion or portions of the image        representing elements of the corresponding scene to which an        areal density greater than 50%, preferably greater than 80% of        the target areal density is assigned.        -   The construction of the 3D elementary model preferably            comprises the discretization of the scene in the form of a            cloud of points. The discretization of the scene is            preferably carried out by photogrammetric processing of the            high-resolution portions of the images representing elements            included in the scene. Preferably, the photogrammetric            processing includes the digital correlation of            high-resolution portions of several images, for example of            the entirety of the images acquired at the corresponding            location. It may be carried out using at least two images            respectively acquired at consecutive acquisition locations            on the path. For example, the photogrammetric processing is            implemented by means of the software application PhotoScan            published by the company Agisoft. The assembly of the 3D            elementary models for producing the continuous 3D digital            model may be implemented in the same way.

The continuous 3D model is preferably formed from a cloud of points. Avoxel may be defined at each point of the cloud. A voxel is a volume ofparallelepipedic, preferably cubic, shape. The size of the voxelcorresponds to the diameter of the smallest sphere circumscribed on thevoxel. The voxel represents a region of the object represented by one ormore pixels in high-resolution portions of images. The size of the voxeldepends on the resolution of the corresponding pixels.

The invention lastly relates to a data storage unit comprising a bank ofimages obtained by the acquisition method according to the invention.The data storage unit may be a hard disk or SSD, a flash memory, forexample of a USB stick.

The invention will be better understood upon reading the detaileddescription that follows of non-limiting examples of implementation ofthe latter, and upon examining the appended drawing, in which:

FIG. 1 shows schematically an element to be imaged, a lens and a sensorof a camera,

FIG. 2 illustrates an environment to be acquired according to aperspective view,

FIG. 3 shows a view in the vertical direction V of the environment inFIG. 3, and

FIGS. 4 and 5 are images of the scenes of the environment, acquired atvarious acquisition locations.

FIG. 2 shows, by way of illustration, an environment 40 in which abuilding 46 is disposed comprising a wall 45. The wall is curved andcomprises inscriptions in the form of the letters “A” to “C”. The methodaccording to the invention is implemented in order to acquire images ofvarious scenes of the environment.

An acquisition device 50 is positioned along a path the direction oftravel of which is shown by the arrow 51 between first 55 and last 60acquisition locations. For the sake of clarity, only four acquisitionlocations 55, 60 and 61, 62 are shown in FIG. 2, but the path maycomprise a higher number thereof.

The acquisition device preferably comprises several cameras, takingdifferent images of the same scene 63,64. However, for the sake ofclarity, the viewing angle α and the images 70, 75 acquired by a singlecamera 65 are shown.

The acquisition device is positioned in such a manner as to comply withthe condition (2) previously described. In this way, at each acquisitionlocation, it is ensured that the images acquired by the device compriseat least one high-resolution portion.

Thus, as observed in FIG. 4, the portions 80 a-c of the wall shown inFIG. 3, seen by the camera at each acquisition location, which aredisposed at a distance less than or equal to D, are represented in thecorresponding images by high-resolution portions 85 a-c, shown by thehatched area 86 bounded by dashed lines 87. The other portions 90 a-c ofthe images comprise portions in which the maximum density of pixels islower than in the portions 80 a-c. This is because they correspond toportions 95 a-c of the wall situated further away from the camera at thecorresponding acquisition location 61, 62.

Subsequently, a continuous 3D digital model of the environment may begenerated by means of the high-resolution portions. For example, bymeans of a photogrammetric processing of the high-resolution portions 85a and 85 b, a 3D elementary model of the portion of the wall comprisingthe letter “B” may be generated.

When the 3D digital model is displayed, for example by means of anaugmented reality headset, the observer can virtually move around themodeled region of the wall comprising the letter “B” and observe withinit the same level of detail irrespective of the level of observation.

It goes without saying that the image acquisition method may beimplemented for acquiring images of an environment more complex thanthat illustrated in the drawing. The invention is not limited to theembodiments presented hereinabove.

The invention claimed is:
 1. A method of acquiring images comprisingmoving, into a plurality of acquisition locations, of an acquisitiondevice comprising at least one camera, and acquisition at eachacquisition location of at least one image of a scene by the camera,each acquisition location being chosen in such a manner that: scenesviewed by the camera in two consecutive acquisition locations andcorresponding images overlap, at least partially, and areal density ofpixels assigned to at least one element of the corresponding scene,which is represented in the corresponding image by a high-resolutionportion, is greater than 50% or greater than 80% of a target arealdensity, the areal density of pixels being defined as a ratio of thearea of the element projected in a plane perpendicular to an opticalaxis of the camera over a quantity of pixels of the high-resolutionportion, the target areal density being greater than or equal to 50px·mm⁻² (pixels per square millimeter).
 2. The method according to claim1, the target areal density being greater than or equal to 60 px·mm⁻²,or in a range between 70 px·mm⁻² and 100 px·mm⁻².
 3. The methodaccording to claim 2, wherein focusing of the camera is identical at allthe acquisition locations and/or the lighting of the scene has aconstant intensity at all the acquisition locations.
 4. The methodaccording to claim 1, wherein the acquisition device is moved along apath defined by the consecutive acquisition locations, then, startingfrom the last acquisition location of the path, the acquisition deviceis moved in a direction of the first acquisition location of the path,in such a manner as to acquire scenes different from those acquiredwhile following the path.
 5. The method according to claim 1, theacquisition device comprising at least two cameras, or at least fivecameras, or at least ten cameras, or at least fifteen cameras, thecameras each acquiring an image of the corresponding scene along aviewing axis different from the others, the images thus acquired of asame scene overlapping one another.
 6. The method according to claim 1,wherein the acquisition device is moved between first and lastacquisition locations, a number of stops between the first and lastacquisition locations is less than 10 or less than 5, and the movementof the acquisition device between the first and last acquisitionlocations is carried out without stopping.
 7. The method according toclaim 1, wherein the acquisition device is moved along a path defined bythe consecutive acquisition locations, the path comprising more than 10acquisition locations per kilometer, or more than 100 acquisitionlocations per kilometer, or more than 1000 acquisition locations perkilometer, or even more than 10,000 acquisition locations per kilometer,or even more than 100,000 locations per kilometer, depending on lengthof the path.
 8. The method according to claim 1, wherein the acquisitiondevice is moved along a path defined by the consecutive acquisitionlocations, the path comprising more than 1000 acquisition location persquare kilometer, or more than 10,000 acquisition locations per squarekilometer, or more than 1 million acquisition locations per squarekilometer, or even more than 10 million acquisition locations per squarekilometer and a surface on which the acquisition locations are situatedis greater than 1 m², or greater than 100 m², or greater than 1000 m²,or greater than 1000 m².
 9. The method according to claim 1, wherein asame element is acquired at least 5 times, or at least 10 times, or atleast 50 times at various acquisition locations.
 10. The methodaccording to claim 1, wherein an average speed of movement of theacquisition device between the first and last acquisition locations isgreater than 0.4 m·s⁻¹.
 11. The method according to claim 1, furthercomprising moving into at least 10 acquisition locations per minute, orinto at least 60 acquisition locations per minute, or into at least 100acquisition locations per minute.
 12. The method according to claim 1,wherein a frequency of acquisition of the images during the movement isgreater than 0.5 Hz or greater than 24 Hz.
 13. The method according toclaim 1, the high-resolution portion representing more than 20% of thetotal number of pixels defining the image, or less than 100% of thetotal number of pixels defining the image.
 14. A method for constructinga 3D digital model representing at least one object, comprisingacquisition of a plurality of images between first and last acquisitionlocations by the method according to claim 1; selection, for each sceneacquired, of at least one high-resolution portion in at least onecorresponding image; construction of a 3D elementary model of each sceneacquired by means of the high-resolution portion; and assembly of the 3Delementary models in order to form the continuous 3D digital modelbetween the first and last acquisition locations.
 15. The methodaccording to claim 14, wherein the selection of the high-resolutionportion or portions of the image comprises: recognition in the image, bycorrelation of images, of areas common to the image and to another imageoverlapping the image at least partially, the other image being acquiredby the camera at another acquisition location or being acquired byanother camera of the acquisition device at the same acquisitionlocation, calculation of the distance between the main object plane ofthe camera such as disposed at the acquisition location of the image andthe elements of the corresponding scene represented on the common areasof the image, according to a stereoscopic calculation algorithm, so asto determine the areal density of the elements of the correspondingscene, and selection of the portion or portions of the imagerepresenting elements of the corresponding scene to which an arealdensity greater than 50% or greater than 80%, of the target arealdensity is assigned.